1. Field of the Invention
The present invention relates to a mine roof bolt anchored in a bore hole by resin bonding, and more particularly to a mine roof bolt bearing a polymer coating layer between a steel bolt and the resin bonding, that yields under loading.
2. Description of Related Art
The roof/ribs of a mine conventionally are supported by tensioning the roof with 4 to 6 foot long steel bolts inserted into bore holes drilled in the mine roof that reinforces the unsupported rock formation above the mine roof. The end of the mine roof bolt may be anchored mechanically to the rock formation by engagement of an expansion assembly on the end of the mine roof bolt with the rock formation. Alternatively, the mine roof bolt may be adhesively bonded to the rock formation with a resin bonding material inserted into the bore hole. A combination of mechanical anchoring and resin bonding can also be employed by using both an expansion assembly and resin bonding material.
A mechanically anchored mine roof bolt typically includes an expansion assembly threaded onto one end of the bolt shaft and a drive head for rotating the bolt. A mine roof plate is positioned between the drive head and the mine roof surface. The expansion assembly generally includes a multi-prong shell supported by a threaded ring and a plug threaded onto the end of the bolt. When the prongs of the shell engage with rock surrounding a bore hole, and the bolt is rotated about its longitudinal axis, the plug threads downwardly on the shaft to expand the shell into tight engagement with the rock thereby placing the bolt in tension between the expansion assembly and the mine roof surface.
When resin bonding material is used, it penetrates the surrounding rock formation to unite the rock strata and to firmly hold the roof bolt within the bore hole. Resin is typically inserted into the mine roof bore hole in the form of a two component plastic cartridge having one component containing a curable resin composition and another component containing a curing agent (catalyst). The two component resin cartridge is inserted into the blind end of the bore hole and the mine roof bolt is inserted into the bore hole such that the end of the mine roof bolt ruptures the two component resin cartridge. Upon rotation of the mine roof bolt about its longitudinal axis, the compartments within the resin cartridge are shredded and the components are mixed. The resin mixture fills the annular area between the bore hole wall and the shaft of the mine roof bolt. The mixed resin cures and binds the mine roof bolt to the surrounding rock.
With certain mining conditions, particularly those found in hard rock mining, the rock formation in the ribs and above the mine roof are susceptible to movement or rock burst as a result of mine-induced seismicity, the excavation of perimeter rock, minor earthquakes, etc. Under dynamic loading caused by rock bursts, the typical mine roof bolts described above are vulnerable to failure. Various mine roof bolts have been designed in an effort to better withstand rock bursts. In particular, mine roof bolts have been designed to yield allowing the bolt to absorb some of the dynamic loading caused by a rock burst. One such design employs a conical-shaped portion at the far end of the bolt, which is designed to pull through the resin or grout in response to excessive forces applied to the bolt. A further design, as disclosed in U.S. Pat. No. 6,984,091, utilizes a bolt having a widened portion of the bolt shaft that is pulled through a longitudinal bore of an anchor member secured within the bore'hole.